1. Field of the Invention
The invention relates to page-based optical data storage. The invention further relates to optical head servo-tracking in page-based optical data storage applications.
2. Background Art
In page-based optical data storage, a page of data is recorded by storing an optically modulated page of data onto the active recording area of an optically sensitive medium. The page is then read by optically interrogating the medium, producing an optically modulated two-dimensional data page that conveys the information that was previously recorded. Detection is performed by imaging the two-dimensional data page onto the two-dimensional active pixel photodetector area of a sensor array. The detector detects the image, and the image is subsequently processed to decode the data. One example of page-based optical data storage is holographic data storage (HDS), where information is recorded as holographic gratings in the optic medium.
As in other optical data storage technologies [for example, compact disks (CD) and digital versatile disks (DVD)], page-based optical data storage also requires the tracking of focus, radial shifts and longitudinal shifts of the media with respect to the read/write optical head. Moreover, the two-dimensional pages are also sensitive to rotations of the two-dimensional page, especially to rotations about the axis perpendicular to the page (azimuthal tracking).
Background information may be found in Geoffrey W. Burr, Holographic Storage, Encyclopedia of Optical Engineering, Marcel Dekker, Inc., 2003; H. Zhang et al., “Multi-layer Optical Data Storage Based on Two-photon Recordable Fluorescent Disk Media,” Proc. of IEEE 18th Symposium on Mass Storage Systems and Technologies, pp. 225-236, San Diego, 2001; Kurt W. Getreuer, Optical Disk Drives—Mechanical Design And Servo Systems, Encyclopedia Of Optical Engineering, Marcel Dekker, Inc., 2003; and Jean Schleipen et al., Optical Heads, Encyclopedia Of Optical Engineering, Marcel Dekker, Inc., 2003.
Page-based optical data storage technologies and, in particular, HDS, have benefitted greatly from the recent development of low cost CMOS active pixel sensors (APS), such as the ones available today in high resolution digital cameras. These sensors, compared to traditional charge-coupled device detectors, present the advantages of lower unit cost (due to the simpler design and manufacturing) and design flexibility provided by the CMOS process, allowing more functionality to be included in the detector chip. However, these sensors have invariably been optimized for imaging applications, not for data storage.
For the foregoing reasons, there is a need for an improved sensor for page-based optical data storage.